This replaces the answer I wrote at 7am (there was the mistake) because I've thought it through since (when I woke up) and it's more complex than I made out.

If you think about a typical passive, the time of the event will be the same in the active and passive verb:

They make paper in Finland : Paper is made in Finland (Permanently true)What you said interested me : I was interested by what you said (past event)

and so on. Also, passives always have a latent by phrase - even when it's not logical to express it (Paper is made in Finland by them) is grammatically possible even though it wouldn't be used.

But, when the past participle is used with an adjectival function - eg I'm trapped / I'm tired - although the meaning is implicitly passive (ie somebody /something did this to me) the equivalent active clause would not express the same time :

The present perfect form expresses a past event (eg shopping) with a present result (being tired). When the result is a state, then that state can be expressed by an adjective.

The problem is that English sometimes uses the past participle to describe a state (tired, exhausted, surprised, interested, excited etc) and sometimes has a completely different word, which we then call an adjective (happy, sad, angry) etc. Sometimes the participle also exists I was angered/saddened and sometimes not * I was happied. Again notice that I was angered/saddened could be followed by by, but angry and sad not.

The Communicative Grammar (Leech and Svartvik) has this to say : "There are many adjectives that have the same form as -ing or -ed participles ... The difference between the adjective and the participle is not always obvious"

I find this unconvincing. I'd prefer to say that participles can function either as part of the verb phrase or adjectivally and that the sentence structure will differ accordingly. The CG gets itself further into the mire when it points out that when these words are "adjectives" they can be modified by an adverb - I was very irritated, I was extremely tired etc. If it were a passive, they argue, then this wouldn't be possible - * Paper is very made in Finland becomes Paper is made a lot in Finland (my example, not theirs) But then they have to admit that you could also add a by phrase after the "adjective" in some cases : I was very interested/irritated by what he said. They then say : "In these "mixed" constructions we cannot say whether the -ed is a participle or an adjective. This to me is like saying : These two animals are exactly the same except for their colour. Let's call the brown one a guinea pig and the white one a cavie. But of course there are some which are brown and white, and then you can't tell if it's a guinea pig or a cavie.

Again, it's surely a matter of whether the participle describes an action (when you have to use a lot) or a state (when it's functioning adjectivally and therefore takes very etc) But to say that it functions adjectivally is not the same as saying that it is an adjective.